For the transmission of an optical WDM signal along long fiber-optic links, optical amplifier modules are required after respective transmission sections. An effective method of additionally amplifying a signal is based on stimulated Raman scattering, in which a pumping signal is launched into the transmission fiber. The pumping signal may, in this case, be generated via a plurality of pumping sources; typically, laser diodes.
The set of wavelengths of the pumping sources is chosen such that all the channels of the WDM signal, usually in the C band and L band (of about 1525 nm to about 1610 nm), are amplified as equally as possible, taking into account the Raman gain spectrum (see “Fiber Optic Communication Systems”, G. P. Agrawal, 2nd edition, p. 381, FIG. 8.11). A channel with a frequency shift of 13.2 THz in relation to a pumping frequency is amplified to the maximum. If there is a smaller or larger frequency difference between a channel and a pumping signal, the channel is amplified less. By using a relatively large number of different pumping wavelengths, all the channels of the WDM transmission signals are amplified more homogeneously. As a result, a flat Raman gain spectrum of all the channel levels is achieved. Such a Raman amplifier is described, for example, in the older patent application with the file number P 10048460.3.
Mach-Zender interferometers, which allow operation for launched powers of up to 2 W, for example, are often used for multiplexing the various pumping wavelengths. This requires a pumping wavelength array with pumping wavelengths that are equidistant from one another. A more detailed description is given in the publication “Namiki et al., Proc. OAA 2000, Quebec, OMB 2, 7-9”. It is possible to use interference filters when multiplexing a small number of pumping wavelengths. In this case, non-equidistant spacings of the pumping wavelengths also are allowable. The power launched in such multiplexers is, however, lower than in the case of Mach-Zender interferometers. In the publication “Kidorf et al., IEEE Phot. Technol. Lett., 11 (1999), 530-532”, there is a description of non-equidistant distribution of the pumping wavelengths, in the case of which a greater concentration of the smaller pumping wavelengths is provided in comparison with larger pumping wavelengths. A corresponding power transfer from small to larger pumping wavelengths is compensated as a result of Raman interaction along the fiber.
Four-wave mixing FWM occurs between the pumping wavelengths in certain types of fiber, particularly in the case of high powers and/or low dispersion in the region of the pumping wavelengths, when use is made of equidistant pumping wavelengths for signal transmission in the C band and L band (i.e., pumping wavelengths between approximately 1420 nm and 1510 nm and signal wavelengths between about 1525 nm and 1610 nm). A detailed description of four-wave mixing is given in “Agrawal, Non-linear Fiber Optics, 1995, p. 404. This arises in particular whenever spectral components of the pumping signals lie near the zero crossing of the dispersion, such as at 1510 nm, of the amplifier fiber. As a result, new frequency components, or what are termed mixing products, are generated in the case of sums or differences of pumping frequencies that are superposed in or outside the spectrum of the pumping source. Consequently, the mixing products likewise can be superposed directly on the WDM signal spectrum in the case of codirectional pumping into the amplifier fiber and cause interference if the higher pumping wavelengths lie near the smaller signal wavelengths. The signal quality, such as the signal-to-noise ratios OSNR of specific channels of the WDM signal, is worsened as a result. Even in the case of contradirectional pumping into the amplifier fiber, (i.e., if pumping channels and the signal channels to be amplified in the amplifier fiber propagate in opposite directions), the mixing products can cause interferences. This is because the mixing products produced initially in the direction counter to the signal channels are reflected by Rayleigh scattering. The reflected components then propagate in the same direction as the signal channels. In this way, spectral components which are superposed on signal channels can be produced. Since the mixing products are likewise themselves amplified by stimulated Raman scattering, their amplitude can increase in such a way that the transmission is greatly impaired.
Previous methods of reducing four-wave mixing, particularly between pumping signals, use a non-equidistant arrangement of the pumping wavelengths, in order that the reduced mixing products lie outside the ranges of the pumping or signal wavelengths. This technique is explained in “Comparisons of Four-Wave Mixing Suppression Techniques in a Multichannel Coherent WDM Systems”, Bohyeon Hwang, Information and Communication Division, Samsung SDS Co. Ltd., APCC 1997, pages 648-652.
The use of an amplifier fiber with high dispersion also makes suppression of the mixing products from the pumping signals possible. However, a high dispersion is not always desired for the signal channels.
By introducing differential time delays between the channels, with division of the channels and with the use of delay lines of lengths differing from channel to channel, mixing products are also suppressed. However, this only can be achieved by expanding the WDM transmission system in a complex procedure.
German Patent Application DE 10111491.5 also discloses a pumping source with, in each case, a number of pumping lasers for the Raman amplification of a WDM signal with minimized four-wave mixing. In this case, the pumping wavelengths are chosen such that mixing products in the spectrum of a transmitted WDM signal are minimized or removed. This produces defined transmission bands of the WDM signal in the C band and L band with mixing products in between, which consequently do not impair the transmission in the transmission bands.
WO 01/22627 A1 likewise discloses an arrangement for and a method of reducing four-wave mixing in the transmission of a non-return-to-zero (NRZ) WDM signal. The phase of the channels is periodically modulated from channel to channel in such a way that mixing products due to four-wave mixing between the signal channels are suppressed at the transmitter of the transmission system. However, nothing is suggested about mixing products which impair the transmission properties from pumping sources up to launching into the transmission fiber.
It is, therefore, an object of the present invention to eliminate or at least greatly minimize the influence of the mixing products produced from a broadband pumping source due to four-wave mixing.